Fuel injectors useful for such applications as gas turbine combustion engines, direct pressurized fuel from a manifold to one or more combustion chambers. Fuel injectors also function to prepare the fuel for mixing with air prior to combustion. Each injector typically has an inlet fitting connected to the manifold, a tubular extension or stem connected at one end to the fitting, and one or more spray nozzles connected to the other end of the stem for directing the fuel into the combustion chamber. A fuel passage (e.g., a tube or cylindrical passage) extends through the stem to supply the fuel from the inlet fitting to the nozzle. Appropriate valves and/or flow dividers can be provided to direct and control the flow of fuel through the nozzle. The fuel injectors are often placed in an evenly-spaced annular arrangement to dispense (spray) fuel in a uniform manner into the combustor chamber. Additional concentric and/or series combustion chambers each require their own arrangements of nozzles that can be supported separately or on common stems. The fuel provided by the injectors is mixed with air and ignited, so that the expanding gases of combustion can, for example, move rapidly across and rotate turbine blades in a gas turbine engine to power an aircraft, or in other appropriate manners in other combustion applications.
A fuel injector typically includes one or more heat shields surrounding the portion of the stem and nozzle exposed to the heat of the combustion chamber. The heat shields are considered necessary because of the high temperature within the combustion chamber during operation and after shut-down, and prevent the fuel from breaking down into solid deposits (i.e., "coking") which occurs when the wetted walls in a fuel passage exceed a maximum temperature (approximately 400.degree. F. (200.degree. C.) for typical jet fuel). The coke in the fuel nozzle can build up and restrict fuel flow through the fuel nozzle rendering the nozzle inefficient or unusable.
One particularly useful heat shield assembly is shown in Stotts, U.S. Pat. No. 5,598,696, owned by the assignee of the present application. This heat shield assembly includes a pair of U-shaped heat shield members secured together to form an enclosure for the stem portion of the fuel injector. At least one flexible clip member secures the heat shield members to the injector at about the midpoint of the injector stem. The upper end of the heat shield is sized to tightly receive an enlarged neck of the injector to prevent combustion gas from flowing between the heat shield members and the stem. The clip member thermally isolates the heat shield members from the injector stem. The flexibility of the clip member permits thermal expansion between the heat shield members and the stem during thermal cycling, while minimizing the mechanical stresses at the attachment points.
Another useful stem and heat shield assembly is shown in Pelletier, U.S. patent application Ser. No. 09/031,871, filed Feb. 27, 1998, and also owned by the assignee of the present application. In this heat shield assembly, the fuel tube is completely enclosed in the injector stem such that a stagnant air (dry territory) gap is provided around the tube. The fuel tube is fixedly attached at its inlet end and its outlet end to the inlet fitting and nozzle, respectively, and includes a coiled or convoluted portion which absorbs the mechanical stresses generated by differences in thermal expansion of the internal nozzle component parts and the external nozzle component parts during combustion and shut-down.
Many fuel tubes also require secondary seals (such as elastomeric seals) and/or sliding surfaces to properly seal the heat shield to the fuel tube during the extreme operating conditions occurring during thermal cycling.
While such heat shield assemblies as described above are useful in certain applications, they require a number of components, and additional manufacturing and assembly steps, which can increase the overall cost of the injector, both in terms of original purchase as well as a continuing maintenance. In addition, the heat shield assemblies can take up valuable space in and around the combustion chamber, block air flow to the combustor, and add weight to the engine. This can all be undesirable with current industry demands requiring reduced cost, smaller injector size ("envelope") and reduced weight for more efficient operation.
Because of limited fuel pressure availability and a wide range of required fuel flow, many fuel injectors include pilot and secondary nozzles, with only the pilot nozzles being used during start-up, and both nozzles being used during higher power operation. The flow to the secondary nozzles is reduced or stopped during start-up and lower power operation. Such injectors can be more efficient and cleaner-burning than single nozzle fuel injectors, as the fuel flow can be more accurately controlled and the fuel spray more accurately directed for the particular combustor requirement. The pilot and secondary nozzles can be contained within the same nozzle stem assembly or can be supported in separate nozzle assemblies. Dual nozzle fuel injectors can also be constructed to allow further control of the fuel for dual combustors, providing even greater fuel efficiency and reduction of harmful emissions.
As should be appreciated, fuel injectors with pilot and secondary nozzles require complex and sophisticated routing of the fuel to the spray orifices in the nozzle. The fuel not only has to be routed through the nozzle portion of the fuel injector, but also through the stem. Such routing becomes all the more complex in multiple nozzle arrangements, where multiple nozzles are fed along a common stem. The routing also becomes more complex if cooling circuits are included to cool the nozzle portion of the injector.
A typical technique for routing fuel through the stem portion of the fuel injector is to provide concentric passages within the stem, with the fuel being routed separately through different passages. The fuel is then directed through passages and/or annular channels in the nozzle portion of the injector to the spray orifice(s). Mains, U.S. Pat. No. 5,413,178, for example, which is also owned by the assignee of the present application, shows concentric passages where the pilot fuel stream is routed down and back along the secondary nozzle for cooling purposes. This can also require a number of components, and additional manufacturing and assembly steps, which can all be contrary to the demands of cost reduction and weight, and small injector envelope.
With current trends toward developing even more efficient and cleaner-burning combustors, it is a continuing challenge to develop improved fuel injectors to properly deliver fuel to a combustion chamber for operation of the gas turbine engine, and which will fit into a small envelope, have a reduced weight, fewer components, and can be manufactured and assembled in an economical manner.